The purpose of the Wind Scatterometer is to obtain information on
wind speed and direction at the sea surface for incorporation into
models, global statistics and climatological datasets. It operates
by recording the change in radar reflectivity of the sea due to the
perturbation of small ripples by the wind close to the surface.
This is possible because the radar backscatter returned to the
satellite is modified by wind-driven ripples on the ocean surface
and, since the energy in these ripples increases with wind
velocity, backscatter increases with wind velocity.

The three antennae generate radar beams looking 45deg. forward,
sideways, and 45deg. backwards with respect to the satellite's
flight direction. These beams continuously illuminate a 500 km wide
swath (see the figure) as the satellite moves along its orbit. Thus
three backscatter measurements of each grid point are obtained at
different viewing angles and separated by a short time delay. These
"triplets" are fed to a mathematical model which calculates surface
wind speed and direction. The main technical characteristics of the
Wind Scatterometer are listed below:

Spatial resolution

>=45 km (along and across track)

Radiometric resolution (4 m/sec)

<=8.5% (mid beam)
<=9.7% (fore/aft beam)

Radiometric resolution (24 m/sec)

<=6.5% (mid beam)
<=7.0% (fore/aft beam)

Radiometric stability

CMIS: <=0.57 dB
IIS: <=0.46 dB

Cross polarisation

>=15 dB

Swath width

>=500 km

Swath stand-off

200 km to right of sub-satellite track

Localisation accuracy

+-5 km (along and across track)

Wind direction range/accuracy

0 - 360deg. / +-20deg.

Wind speed range/accuracy

4 m/s - 24 m/s / 2 m/s or 10 %

A transmit pulse is produced by the Scatterometer Electronics
and amplified by the IF Radar unit, converted to an RF signal in
the transmitter/converter unit and amplified by the High-Power
Amplifier. The transmit signal is routed to the correct antenna by
the Circulator Assembly which in this mode is under the control of
the Scatterometer Electronics.

The received signal is down-converted, amplified by the IF Radar
and routed to the Scatterometer Electronics. A measurement sequence
of 3.763 seconds (see the figure) corresponds to 25 km along the
sub-satellite track at a satellite altitude of 785 km and is
continuously repeated in the wind mode without any gap. This
sequence involves four sets of measurements, regularly spaced, for
each antenna beam (fore, mid and aft). Each series corresponds to
32 measurement pulses on each beam. Noise measurements and internal
calibration are regularly performed in the interval between the
transmitted pulse and the reception of the return echo.

For the mid-beam, the return echo is filtered and sampled in
complex form I and Q, while for the fore and aft-beams, as the
doppler variation is significant over the swath width (20 KHz near
swath to 140 KHz far swath), a programmable doppler compensation
law is applied to the received signal before filtering and complex
sampling.